Bird Density Estimates Research Articles

In the bleak mid-winter: The value of brownfield sites for birds during the winter

Brownfield sites (e.g., wastelands, disused industrial grounds and power stations) are valuable refuges for urban birds during the breeding season. This owes to the presence of vegetation in different stages of succession, which provide a high diversity of food and breeding resources. However, insufficient research has explored their winter value, when temperatures are lower and food is scarcer. Accordingly, 342 bird and habitat surveys were conducted in brownfield sites, and other land-uses, across Greater Manchester. Bird density estimates were then calculated using Distance software and bird-habitat associations were analysed using Canonical Correspondence Analysis (CCA). Moreover, the impact of brownfield site development on winter bird density from 2000 to 2009 in Greater Manchester was explored using the bird density estimates, and historical image function in Google Earth. In brownfield sites, Pica pica had the highest density estimate (142 per km2), although four species had estimated densities > 100 per km2 (Turdus merula; Cyanistes caeruleus; Passer domesticus; Aegithalos caudatus). The CCA indicated that Aegithalos caudatus, Cyanistes caeruleus, Parus major, Prunella modularis were associated with Fagus sylvatica, Betula pendula, and Buddleja davidii on brownfield sites, which during the winter 1) produce seeds 2) support overwintering insects, providing a valuable food source. Nevertheless, across land-uses, most species favoured green space and pre-2000 housing. Brownfield sites were most replaced between 2000 and 2009, with 17.8 km2, 14.9 km2, and 12 km2 replaced by commercial, industrial, and post-2000 housing respectively. This is likely to have had a negative impact on the winter density of most bird species, owing to 1) the removal of brownfield site vegetation 2) increases in impervious land cover. Future declines in winter bird abundance could be mitigated by retaining brownfield site vegetation and setting aside brownfield sites in close proximity to green space and pre-2000 housing.

Estimating bird detection distances in sound recordings for standardizing detection ranges and distance sampling

Abstract Autonomous sound recorders are increasingly used to survey birds, and other wildlife taxa. Species richness estimates from sound recordings are usually compared with estimates obtained from established methods like point counts, but so far the comparisons were biased: Detection ranges usually differ between the survey methods, and bird detection distance data are needed for standardizing data from sound recordings. We devised and tested a method for estimating bird detection distances from sound recordings, using a reference recording of test sounds at different frequencies, emitted from known distances. We used our method to estimate bird detection distances in sound recordings from tropical forest sites where point counts were also used. We derived bird abundance and richness measures and compared them between point counts and sound recordings using unlimited radius and fixed radius counts, as well as distance sampling modelling. First we show that it is possible to accurately estimate bird detection distances in sound recordings. We then demonstrate that these data can be used to standardize the detection ranges between point counts and sound recordings with a fixed‐radius approach, leading to higher abundance and richness estimates for sound recordings. Our distance‐sampling approach also revealed that sound recorders sampled significantly higher bird densities than human point counts. We show for the first time that it is possible to standardize detection ranges in sound recordings and that distance sampling can successfully be used too. We revealed that birds were flushed by human observers and that this possibly leads to lower density estimates in point counts, although sound recorders could also have sampled more birds because of their earlier deployment times. Sound recordings are more amenable to distance‐sampling modelling than point counts as they do not exhibit an observer‐induced avoidance effect, and they can easily collect more replicates for obtaining more accurate bird density estimates. Quantifying bird detection distances was so far one important shortcoming that hindered the adoption of modern autonomous sound recording methods for ecological surveys.

Modelling future scenarios to improve woodland landscapes for birds in the Mediterranean

Agro-silvo-pastoral systems occupy much of the Mediterranean region. Promoting landscape heterogeneity in these systems is a potentially valuable management strategy to enhance woodland biodiversity. We evaluated the effects on birds of managing patches of olive groves and riparian galleries to promote heterogeneity of these systems in Portugal.For a large cork oak woodland we (i) used nesting season bird density estimates and habitat descriptors to generate suitability models for each studied species; (ii) identified areas suitable to establish new patches of riparian galleries and olive groves, and used this information to (iii) generate scenarios maps with different levels of cover of both habitats. Finally, (iv) the model of each bird species was applied over the future scenarios to estimate potential changes in its distribution and abundance.Future scenarios with greater availability of riparian vegetation resulted in major increases in projected populations of seven of the 21 studied species, but gains with increases of olive groves were predicted in just three species. Bird species predicted to increase the most with the recovery of riparian vegetation are presently among the least abundant in the region. Restoring riparian galleries would be highly beneficial for birds, whereas promoting traditional olive groves would have lower conservation implications.The methodology used proved helpful to select areas particularly appropriate for habitat management, generate simulated landscapes incorporating this management, and predict the population and spatial responses of species to these simulated scenarios, thus having a good potential to guide conservation management. Moreover, our results indicate that simple management measures should have substantial positive effects on the bird community of oak woodlands, without compromising the essential economic return of the system.

Mapping seabird sensitivity to offshore wind farms.

We present a Geographic Information System (GIS) tool, SeaMaST (Seabird Mapping and Sensitivity Tool), to provide evidence on the use of sea areas by seabirds and inshore waterbirds in English territorial waters, mapping their relative sensitivity to offshore wind farms. SeaMaST is a freely available evidence source for use by all connected to the offshore wind industry and will assist statutory agencies in assessing potential risks to seabird populations from planned developments. Data were compiled from offshore boat and aerial observer surveys spanning the period 1979–2012. The data were analysed using distance analysis and Density Surface Modelling to produce predicted bird densities across a grid covering English territorial waters at a resolution of 3 km×3 km. Coefficients of Variation were estimated for each grid cell density, as an indication of confidence in predictions. Offshore wind farm sensitivity scores were compiled for seabird species using English territorial waters. The comparative risks to each species of collision with turbines and displacement from operational turbines were reviewed and scored separately, and the scores were multiplied by the bird density estimates to produce relative sensitivity maps. The sensitivity maps reflected well the amassed distributions of the most sensitive species. SeaMaST is an important new tool for assessing potential impacts on seabird populations from offshore development at a time when multiple large areas of development are proposed which overlap with many seabird species’ ranges. It will inform marine spatial planning as well as identifying priority areas of sea usage by marine birds. Example SeaMaST outputs are presented.

Science-Driven Management of Protected Areas: A Philippine Case Study

The lack of scientific baseline information hinders appropriate design and management of protected areas. To illustrate the value of science to management, we consider five scenarios for the 202.0 km² Puerto Princesa Subterranean River National Park, Philippines: (1) closure to human activities, (2) and (3) two levels of increase in unplanned human activities, (4) creation of a forest corridor and (5) additional allocation of land for permanent or shifting agriculture. We then use habitat-specific bird density estimates to simulate the net effect of each scenario on 18 focal bird populations. Closure has significant benefits-populations of five species are predicted to increase by >50 % and nine by >25 %, but two secondary forest flycatchers, including the endemic and 'Vulnerable' Palawan flycatcher, decline dramatically, while the creation of a 4.0 km² forest corridor yields average increases across species of 2 ± 4 % (SD). In contrast, heavier unplanned park usage produces declines in all but a few species, while the negative effects of an extra 2.0 km² of shifting cultivation are 3-5 times higher than for a similar area of permanent agriculture and affect species whose densities are highest in primary habitats. Relatively small changes within the park, especially those associated with agricultural expansion, has serious predicted implications for local bird populations. Our models do not take into account the full complexities of bird ecology at a site, but they do provide park managers with an evidence base from which to make better decisions relating to biodiversity conservation obligations which their parks are intended to meet.

Changes in patch features may exacerbate or compensate for the effect of habitat loss on forest bird populations.

One and a half centuries after Darwin visited Chiloe Island, what he described as “…an island covered by one great forest…” has lost two-thirds of its forested areas. At this biodiversity hotspot, forest surface is becoming increasingly fragmented due to unregulated logging, clearing for pastures and replacement by exotic tree plantations. Decrease in patch size, increased isolation and “edge effects” can influence the persistence of forest species in remnant fragments. We assessed how these variables affect local density for six forest birds, chosen to include the most important seed dispersers (four species) and bird pollinators (two species, one of which acts also as seed disperser), plus the most common insectivore (Aphrastura spinicauda). Based on cue-count point surveys (8 points per fragment), we estimated bird densities for each species in 22 forest fragments of varying size, shape, isolation and internal-habitat structure (e.g. tree size and epiphyte cover). Bird densities varied with fragment connectivity (three species) and shape (three species), but none of the species was significantly affected by patch size. Satellite image analyses revealed that, from 1985 to 2008, forested area decreased by 8.8% and the remaining forest fragments became 16% smaller, 58–73% more isolated and 11–50% more regular. During that period, bird density estimates for the northern part of Chiloé (covering an area of 1214.75 km2) decreased for one species (elaenia), increased for another two (chucao and hummingbird) and did not vary for three (rayadito, thrust and blackbird). For the first three species, changes in patch features respectively exacerbated, balanced and overcame the effects of forest loss on bird population size (landscape-level abundance). Hence, changes in patch features can modulate the effect of habitat fragmentation on forest birds, suggesting that spatial planning (guided by spatially-explicit models) can be an effective tool to facilitate their conservation.

Quantifying Bird Density During Migratory Stopover Using Weather Surveillance Radar

Increasingly, data from weather surveillance radars are being used by biologists investigating the ecology and behavior of birds, insects, and bats in the aerosphere. Unfortunately, these radars quantify echoes caused by layered biological targets such as migrating birds in a manner that introduces bias in radar measures. We investigated the performance of a bias-adjustment algorithm that adjusts radar measures for vertical variation of reflectivity, nonstandard beam refraction, and spatial displacement of radar targets. We evaluated the efficacies of four variations of this algorithm by their ability to increase correspondence between radar reflectivity measured at two weather radar sites and the ground density of migrating birds measured during two autumn seasons and two spring seasons among 24 hardwood forest sites along the northern coast of the Gulf of Mexico. The algorithm integrated close-range reflectivity data from the five lowest elevation angle sweeps to derive high-resolution vertical profiles of reflectivity (VPRs) that closely corresponded to the observed vertical target density profiles based on a vertically oriented portable radar. The radar reflectivity of birds aloft near the onset of migratory flight was positively correlated with the bird density on the ground. All four radar data adjustment schemes that we tested produced significant improvement in the accuracy of bird density estimates relative to unadjusted radar data. In general, adjusting reflectivity based solely on the VPRs derived using observed refractive conditions yielded the most accurate radar-based estimates of bird density.

Assessment of census techniques for interspecific comparisons of tropical rainforest bird densities: a field evaluation in the Western Ghats, India

Studies comparing different bird censusing methods are useful for assessing relative biases, synthesizing data across studies, and designing bird population monitoring programmes. A field study was carried out in mid‐elevation tropical rainforest in the Western Ghats to compare bird density estimates from line transect, point count and territory spot‐mapping methods. Interspecific comparisons were made using data for 13 common resident bird species, including two endemics. Variable‐width line transect density estimates were highly correlated with, but slightly (17%) higher than, those produced by territory spot‐mapping. Although densities from variable‐width point counts and spot‐mapping were highly positively correlated, the estimates were 95% higher on average in the former. Higher density estimates relative to spot‐mapping were produced mainly for the most abundant species, probably due to their mobility and the inclusion of additional individuals that enter the count area during the count period. Fixed‐width strip transects and point counts produced density estimates that were highly correlated with, but significantly lower than, variable‐width estimates. Wherever possible, territory spot‐mapping and line transects are recommended for density estimates; the former may yield additional information on spatial distribution of birds. Fixed‐width transects or point counts, being easier to apply, may be used for large‐scale monitoring programmes. Interspecific variation in flocking systems and the poor visibility in dense rainforest vegetation indicate the need for care in collection of data on flock size and its variation, which is necessary for estimating the density of individuals. The variation across methods suggests the need for further research using multiple methods across years and marked individuals to verify territoriality and accuracy.

Quantifying the impacts on biodiversity of policies for carbon sequestration in forests

There is currently a great deal of interest in the use of afforestation (conversion of non-forest land to forest) to reduce atmospheric concentrations of carbon dioxide. To date, economic analyses have focused on the costs of forest carbon sequestration policies related to foregone profits from agricultural production. No studies have examined additional costs or benefits associated with impacts on biodiversity. The main objective of this paper is to estimate the changes in farmland and forest bird populations that are likely to occur under an afforestation policy. Econometric models of land use are used to simulate the response of private landowners to subsidies for tree planting on agricultural land. We evaluate subsidies that achieve conversion of 10% of the total agricultural land in each of three U.S. states (South Carolina, Maine, and southern Wisconsin). Bird density estimates are derived for 615 species with data from the national Breeding Bird Survey. Percentage changes in agricultural and forest land for each county are applied to county-level estimates of bird densities for farmland and forest birds. Despite considerable spatial variation in agricultural land conversion rates and farmland bird distributions within these states, statewide losses of farmland birds were relatively uniform at 10.8–12.2%. Increases in forest bird populations, however, varied substantially between states: 0.3% in Maine, 2.5% in South Carolina, and 21.8% in southern Wisconsin. Surprisingly, a net loss in total bird populations results in all three states (−2.0% in Maine, −2.3% in South Carolina, and −1.1% in southern Wisconsin), despite the prevailing wisdom as to bird-rich forests. The loss is due to the coincidence of centers of high farmland bird richness and low forest bird richness with areas economically suited to conversion. Additional gains in forest species may result, however, if afforestation within the economically optimal counties is concentrated to fill in existing forest fragments presently suffering avian losses to edge predators. Our results thus show that assessments of the biological consequences of afforestation for carbon sequestration must consider both current land cover and the distributional patterns of organisms as well as the policy's conversion goal.

Density estimates of birds inhabiting fragments of cloud forest in southern Ecuador

SummaryBetween June and September 1995 estimates were made of the population densities of birds inhabiting fragments of unprotected montane cloud forests in southern Ecuador. The study focused on 10 species: Bearded Guan Penelope barbata, Golden-plumed Parakeet Leptosittaca branickii, Red-faced Parrot Hapalopsittaca pyrrhops, White-capped Parrot Pionus seniloides, Grey-breasted Mountain-Toucan Andigena hypoglauca, Barred Fruiteater Pipreola arcuata, Turquoise Jay Cyanolyca turcosa, Great Thrush Turdus fuscater, Hooded Mountain-tanager Buthraupis montana, and Mountain Cacique Cacicus leucoramphus. Species assemblages and population densities varied greatly between sites, despite the presence of similar habitats and altitudinal ranges. Substantial populations of the threatened Bearded Guan, Grey-breasted Mountain-Toucan and Golden-plumed Parakeet were found in areas previously unstudied, as well as in areas previously known to be important. Currently unprotected areas are important for the conservation of montane cloud forest birds.

Relative Importance of Bird Species as Hosts for Immature Ixodes dammini (Acari: Ixodidae) in a Suburban Residential Landscape of Southern New York State

Abundance of birds and their tick parasites were estimated in a residential community located in Westchester County, NY, where Lyme disease is endemic. In total, 36 bird species (416 captures) were collected, of which 25 species (69%) were parasitized by ticks. Ixodes dammini Spielman, Clifford, Piesman & Corwin composed 96.4% of the 1,067 ticks found on birds. The bird species most heavily parasitized was house wren, Troglodytes aedon Vieillot (11.1 Ixodes dammini per bird). Relative density estimates of birds, using fixed circle radius counts, revealed dominance by the American robin, Turdus migratorius L. (29.3%), followed by the common grackle, Quiscalus quiscula (L.) (9.3%). Bird density estimates were combined with tick abundance data to calculate an importance value for each bird species as a host for immature I. dammini. The American robin was most important, accounting for 72.7% of all larval I dammini found on birds, followed by the common grackle (8.2%) and the house wren (5.9%). Both the American robin and house wren are reservoir competent for the Lyme disease spirochete Borrelia burgdorferi Johnson, Schmid, Hyde, Steigerwalt & Brenner and therefore may contribute to the risk of Lyme disease for humans. During August when larvae were most prevalent, 39% of the American robins and 70% of the common grackles were observed on lawns. These species are probable contributors to nymphal I. dammini populations found on lawns.

A Comparison of Transects and Spot Mapping in Oak-Pine Woodlands of California

We compared density estimates of birds using the spot-mapping and transect methods in grazed and ungrazed habitats in foothill woodlands of the western Sierra Nevada, California. As judged by the percentage error of transect estimates in relation to those from spot mapping, the exponential polynomial estimator from program TRANSECT (Laake et al. 1979) consistently gave better estimates of density than either the Fourier series or exponential power series estimators. Distance estimates grouped into bands of some specified width consistently performed better than ungrouped data, although no single band width was best in all cases. Although several alternatives tested in this study gave density estimates with errors only about half those by a variety of previous methods, even the best alternatives had mean errors of about 25% and maximum errors ranging from 37% to 109%. Errors involved both overand underestimation of density in relation to spot-mapping estimates, even for the same species on the same site in different months or by different observers. The two habitats affected TRANSECT estimates of density differently, and observers differed significantly in the accuracy of their estimates. Because of these problems, transect methods could lead to misinterpretation of real abundance patterns by masking differences between years or sites, even to the extent of suggesting that a species was more abundant in a given year or site than another when the reverse was true. As sample sizes increased, both overestimates and underestimates of density by transect methods converged on density estimates by spot mapping, suggesting a real correspondence in density estimation by these two methods. Empirical assessment of the sample size needed to use program TRANSECT indicated that a data set should include at least 100 records per species--far more than the number normally used to estimate densities by transects.